This proposal is focused on immune control of infections due to orthopoxviruses (OPXVs), a medically relevant genus of large double-stranded DNA viruses that includes variola, the causative agent of smallpox. Although smallpox has been eradicated from natural infections, there is still major concern about its use as a bioweapon. Moreover, zoonotic infections are caused by related OPXVs, including monkeypox virus, and cowpox virus (CPXV). The OPXVs possess open reading frames (ORFs) encoding viral proteins that evade the host immune response. This application centers on CPXV because it is thought to have the largest repertoire of immune evasion genes and is endemic in rodents making CPXV infections in experimental mice appropriate for study of host-pathogen interactions. In published and unpublished studies, the applicants have proof-of-principle data indicating that CPXV investigations can inform topics of general interest regarding immune control of viruses. Therefore, the overall hypotheses are that CPXV encodes novel molecules that evade host immunity and that study of these molecules will reveal novel insights into host mechanisms controlling viruses. To address these hypotheses, the applicants have assembled a group of four highly qualified principal investigators with significant track records in studying OPXVs and CPXV to work on three distinct projects in collaboration. Thus, the overall Specific Aims of this U19 application are: 1) Determine the ORFs in CPXV that thwart the host immune response, particularly those mediated by natural killer (NK) and T cells and assess the outcome of in vivo infections in mice with viruses specifically lacking the immune evasion genes. 2) Determine the mechanism by which these CPXV encoded proteins modulate the immune response by identifying the specific host interacting ligands and receptors. 3) Determine the structural basis for the interaction of CPXV immune evasion proteins with host targets. 4) Determine if CPXV modulates the human immune response. 5) Determine other host immune responses to CPXV. Therefore, this proposal has the potential to lead to advances with clinical significance.